1. Field of the Invention
This invention relates generally to metal components and, more particularly, to a method of sealing an opening in a hollow metal component.
2. Related Art
A turbomachinery blade is typically comprised of a retaining or root section, a platform section and an airfoil section. Since it is desirable that a turbomachine be operated at high speeds and temperatures, the airfoil section of the blade is preferably formed of relatively thin convex and concave sidewalls defining an internal cavity, which has provided therein means for cooling the blade sidewalls. In an air cooled blade, the cooling air is commonly brought in through passageways in the root portion of the blade to the airfoil cavity which is divided by a plurality of ribs into heat transfer chambers and serpentine passageways. In an open cooling path arrangement, the cooling air within the cavity acts to cool the blade sidewalls by a combination of impingement, convection and film cooling, and then is normally discharged out through film holes and trailing edge holes.
Because of the critical aerodynamic shape of the blade, and the required intricate structure in the cavity portion thereof, one practical method of fabrication has been by way of the casting process, wherein precision casting techniques are used to fabricate the turbine blade airfoil and root section. When such an investment casting is used for the turbine blade, it is desirable to use a ceramic or glass core to form the internal chambers and passageways within the airfoil cavity. This is normally facilitated by leaving open the tip end of the airfoil and allowing the ceramic core to extend outwardly therefrom for purposes of handling and positioning the core for location during the fabrication process. It then subsequently becomes necessary to seal the tip end of the blade in order to prevent cooling fluid from being wastefully discharged into the working gas stream. This open tip end of the blade is generally sealed by means of a tip cap, which is joined to the blade with either a metallurgical or mechanical bond.
The opening in the blade tip is commonly referred to as the core print. One method of closing the core print is to braze a relatively thin metal plate of IN625 to the top surface of the blade over the core print opening to seal the opening, subsequent to taking the blade through the hot iso-static pressing step which is employed to eliminate cast defects. The thin metal seal plate is tack-welded around the blade tip core print opening, typically with brazing tape or brazing powder applied in the contact area between the blade tip and the seal plate.
A second method for sealing the tip that is currently employed uses the Liburdi Powder Metallurgy Process described in U.S. Pat. No. 5,156,321. In the application of this process, a land is typically machined in the core print opening, forming a step in the lower surface of the opening just above the cooling chamber. The seal plate is inserted into the opening resting on and supported by the land while bridging the opening. The volume above the IN625 seal plate is filled with a powder of the blade alloy and topped with a melting point depressant. The blade is then thermally cycled to sinter the alloy powder.
Both processes have not been able to produce parts with zero or minimum rejects or parts with tip closure plates that remain in position in service, due to inadequate long term bond strength as a result of the tremendous centrifugal forces and high temperatures experienced at the blade tips during turbine operation.
Accordingly, it is an object of this invention to provide an improved method of sealing the blade tips that will result in fewer defects.
It is an additional object of this invention to provide a new manufacturing method for sealing the blade tips that provide for a stronger metallurgical bond between the seal plate and the blade tip.
It is a further object of this invention to provide such an improved manufacturing method that will reduce the number of manufacturing steps and insure the cooling passages are not blocked as the result of the bonding operation sealing the seal plate to the blade tip.
It is another object of this invention to avoid undesirable effects of melting point depressant, such as boron, on component properties.